Miniature alkaline silver oxide button cells have gained wide commercial acceptance in the battery industry for many applications because they are characterized as being high capacity, small volume electric cells. In other words, they have a high power output and energy per unit weight and unit volume of active cathode material. However, one of the major disadvantages of divalent silver oxide cells is that they discharge at two successive different potentials. This is due to the fact that the active materials of such cells are first divalent silver oxide (AgO) and then monovalent silver oxide (Ag.sub.2 O). Silver oxide cells using monovalent silver oxide as the only active cathode material will have a theoretical unipotential discharge at about 1.57 volts but the capacity in milliampere hours per gram of monovalent silver oxide is substantially lower than the capacity with divalent silver oxide. On the other hand, silver oxide button cells using only divalent silver oxide as the starting active cathode material will discharge at a first potential at about 1.7 volts across a 300-ohm resistor for the first 40 hours of discharge, for example, and then drop to approximately 1.5 volts for the remaining useful discharge life. Thus monovalent silver oxide cells have the advantage of discharging at a single unipotential plateau but with some sacrifice in capacity as compared with divalent silver oxide cells which have the advantage of having a much higher capacity but with the disadvantage of discharging at two successive distinct voltage plateaus. Divalent silver oxide has about 1.9 times more capacity per gram than monovalent silver oxide and about 2 times more capacity per unit volume than monovalent silver oxide.
Many cell or battery applications, particularly in transistorized devices such as hearing aids, watches and the like, require an essentially unipotential discharge source for proper operation and, therfore, cannot effectively use the dual voltage level discharge which is normally characteristic of divalent silver oxide cells.
Consequently, many methods have been proposed for obtaining a unipotential discharge from a divalent silver oxide cell without undue sacrifice in capacity. One method disclosed in U.S. Pat. Nos. 3,615,858 and 3,655,450 entails providing a continuous layer of monovalent silver oxide in physical and electrical contact with a divalent silver oxide pellet. During assembly of the cell, the cathode pellet is disposed against the inner surface of a cathode cup or collector whereupon the layer of monovalent silver oxide physically isolates the divalent silver oxide from contact with the cathode cup so that the sole electronic path for discharge of the divalent silver oxide is through the monovalent silver oxide layer.
In U.S. Pat. No. 3,476,610 a silver oxide battery is disclosed which employs a positive electrode comprised mainly of divalent silver oxide with the addition of monovalent silver oxide present in part as an electrolyte-impermeable masking layer. This layer isolates the divalent silver oxide from contact with the electrolyte of the battery until discharge begins whereupon the monovalent silver oxide becomes electrolyte-permeable. When this occurs, the electrolyte then begins to contact the divalent silver oxide. In addition, the monovalent silver oxide is also present as an interposed layer between the divalent silver oxide and the inner surface of the cathode cup or collector so as to isolate the divalent silver oxide from electronic contact with said cathode cup which is the positive terminal of the cell.
In U.S. Pat. No. 3,484,295 a silver oxide battery is disclosed which utilizes a positive silver oxide electrode comprising divalent silver oxide and monovalent silver oxide. The latter oxide is employed as an electrolyte-impermeable layer which is interposed between the divalent silver oxide and the battery components containing the electrolyte so as to isolate the divalent silver oxide from contact with the electrolyte until the monovalent silver material is discharged. If the discharge product of the monovalent silver material is continually reoxidized by the divalent silver material in the presence of the battery electrolyte, then it is possible that the battery will yield a unipotential discharge.
In U.S. Pat. No. 3,920,478 a silver oxide cell is disclosed which employs a positive electrode comprising divalent silver oxide housed in a positive electrode container and interposed between the positive electrode and the inner wall of the cathode container and/or between the positive electrode and the separator is a discontinuous oxidizable metal, such as a zinc screen, which functions to isolate a portion of the positive electrode from the container so as to produce a unipotential discharge on low drain conditions.
In U.S. Pat. No. 3,925,102 a silver oxide cell is disclosed which employs a positive electrode comprising divalent silver oxide housed in a positive electrode container having an upstanding wall and a closed end. Interposed between the positive electrode and the inner upstanding wall is an oxidizable zinc ring which functions to isolate a portion of positive electrode from the container so as to produce a unipotential discharge on low drain conditions.
In addition to the disadvantage of a divalent silver oxide electrode in an alkaline silver oxide cell discharging at two successive different potentials, it is relatively unstable when in contact with aqueous alkaline solutions. Specifically, divalent silver oxide evolves oxygen when in contact with aqueous alkaline solutions which results in a loss of capacity due to the conversion of divalent silver oxide to monovalent silver oxide. In addition, the gassing of the divalent silver oxide creates a problem in providing proper sealing of the cells. U.S. Pat. No. 3,853,623 discloses the use of a gold additive admixed into a divalent silver oxide electrode or incorporated into an aqueous alkaline electrolyte of a cell, to improve the stability of the divalent silver oxide electrode in the aqueous alkaline electrolyte.
Accordingly, it is an object of the present invention to provide a silver oxide electrode for electrochemical cells which comprises a major portion of divalent silver oxide and a minor amount of zinc oxide.
Another object of this invention is to incorporate a minor amount of zinc oxide into a divalent silver oxide-containing material so as to produce an improved positive electrode for miniature alkaline silver oxide button cells.
Another object of this invention is to provide a silver oxide cell which employs a positive electrode comprising divalent silver oxide and has a substantially unipotential discharge plateau over the useful life of the cell.
Another object of this invention is to provide a silver oxide electrode for a silver oxide/zinc cell wherein said electrode comprises a major portion of divalent silver oxide and a minor amount of zinc oxide, said zinc oxide being added primarily to diminish the duration of the divalent step during discharge of the cell and to improve the chemical stability of the divalent silver oxide in contact with an aqueous alkaline electrolyte.
Another object of this invention is to incorporate a minor amount of zinc oxide into a divalent silver oxide-containing material so as to improve lubricity of the mixture so that during a pelletizing operation of the mixture, the pellets so formed can be more easily released from the mold.